Well-known commercial processes for the production of monomers, such as unsaturated carboxylic acids and unsaturated nitriles, typically start with one or more alkenes and convert them, by catalytic vapor phase oxidation, to the desired monomer products. In view of the pressures exerted by competition in the industry, and the price difference between alkanes and their corresponding alkenes, such as propane and propene, respectively, efforts are being made to develop processes in which an alkane is used as the starting material to, ultimately, produce the desired monomers at a lower overall cost.
One process modification, which has enjoyed some success in commercial industry, is to simply add an upstream reaction stage in which an alkane is first converted to the corresponding alkene, in the presence of a suitable catalyst. The resulting alkene (e.g., propene) is then fed to the customary oxidation reaction stages, for oxidation of the alkene (e.g., first to acrolein and then to the desired monomer product, as in the two-step oxidation of propene to form acrylic acid). For example, both European Patent Application No. EP0117146 and U.S. Pat. No. 5,705,684 describe multi-stage catalytic processes for converting an alkane (propane) to the corresponding unsaturated carboxylic acid (acrylic acid) which includes an initial alkane-to-alkene conversion stage having one or more suitable catalysts to produces a product stream comprising alkene, which is fed to one or more downstream oxidation stages. Various catalysts and methods are known to catalyze conversion of alkanes to their corresponding alkenes.
For example, mixed metal oxide catalysts having, as essential elements, Mo—Sb—W or Cr—Sb—W, and at least one metal selected from the group consisting of V, Nb, K, Mg, Sn, Fe, Co and Ni, were shown to be useful for oxidative dehydrogenation of propane to produce propene, in single-pass yields of greater than 10% (U.S. Pat. No. 6,239,325). A Pd—Cu/Mn catalyst on zirconium oxide support also catalyzed the oxidative dehydrogenation of ethane, with selectivities to ethane in the range of 70%-80% and diminished coke formation (US Patent Application Publication No. 2005/0124840). Furthermore, oxidative dehydrogenation of ethane in the presence of an Mo—V—Te—Nb-based mixed metal oxide catalyst has been shown to produce ethene in yields as high as 50%, and, in one case, even greater than 60% (US Patent Application Publication No. 2005/0085678). A vanadium-aluminum based mixed metal oxide catalyst, with or without one or more additional metal oxides of Cr, Zn, Fe and Mg, is known to be capable of catalyzing the conversion of propane, n-butane, isobutane, isopentane to their corresponding alkenes, in the presence of oxygen, to achieve relatively high alkene selectivities while minimizing the formation of coke and, thereby, minimizing the need for catalyst regeneration (U.S. Pat. No. 4,046,833). Zhaorigetu, et al. (1996) demonstrated that oxidative dehydrogenation of propane over an unsupported vanadium-based catalyst promoted with one or more rare earth metals (La, Ce, Pr, Sm and Er) could be enhanced by providing carbon dioxide, in addition to oxygen, to the reaction zone (Zhaorigetu, B.; Kieffer, R.; Hinderman J.-P., “Oxidative Dehydrogenation of Propane on Rare Earth Vanadates. Influence of the Presence of CO2 in the feed.” Studies in Surface Science and Catalysis, 1996, 101, 1049-1058).
Since it is exothermic, when an oxidative dehydrogenation process is operated continuously, excess heat must be continuously removed, which increases capital and operating expenditures. Another disadvantage of oxidative dehydrogenation is that selectivity to alkene tends to decrease when the process is operated at higher, commercially useful alkane conversion rates. Thus, in practice, these processes tend to be operated at lower conversion rates (well below 100%), which limits their product yield capacity and generally renders them economically unsuitable for use in commercial-scale processes.
Other catalysts are known to catalyze the endothermic dehydrogenation of an alkane, in the presence of a “weak” oxidant, such as steam or carbon dioxide, to form the corresponding alkene. Some endothermic dehydrogenation catalysts perform better in the absence of oxygen, while others tolerate the presence of minor amounts of oxygen, along with the weak oxidant, without significant loss of activity. Supported vanadium-based catalysts, promoted with Li, Na, K or Mg, have been shown to dehydrogenate ethylbenzene in the presence of a “soft oxidant,” i.e., carbon dioxide, to produce styrene with selectivities of about 98-99%, in the absence of oxygen (Li, X.—H.; Li, W.—Y.; Xie, K.—C., “Supported Vanadia Catalysts for Dehydrogenation of Ethylbenzene with CO2,” Catalyst Letters, December 2005, Vol. 105, Nos. 3-4). Carbon dioxide was provided in varying amounts by Dury, et al. in 2002 to the oxidative dehydrogenation of propane to form propene in the presence of nickel-molybdenum-based catalysts, and found to increase conversion (by about 18-28%) but decrease selectivity (Dury, F.; Gaigneaux, E. M., Ruiz, P., “The Active Role of CO2 at Low Temperature in Oxidation Processes: The Case of the Oxidative Dehydrogenation of Propane on NiMoO4 catalysts,” Applied Catalysis A: General 242 (2003), 187-203). Dury et al. demonstrated that, contrary to the traditional expectation that carbon dioxide is inert in dehydrogenation reactions, carbon dioxide actively participates in the dehydrogenation of propane, even in the absence of oxygen. Takehira, et al. tested the activities of various metal oxide catalysts (Cr, Ga, Ni, V, Fe, Mn and Co) supported on silicon-containing support materials, including mesoporous MCM-41, Cab-O-Sil, and silicon oxide, and found that the Cr-based catalyst supported on MCM-41 provided the best results for dehydrogenation of propane, in the presence of carbon dioxide, to form propene. Takehira, K.; Oishi, Y.; Shishido, T.; Kawabata, T.; Takaki, K.; Zhang, O.; and Wang, Y., “CO2 Dehydrogenation of Propane over Cr-MCM-41 Catalyst,” Studies in Surface Science and Catalysis, 2004, 153, 323-328.
Obviously, endothermic dehydrogenation processes require addition of heat to the process. They typically involve burning (i.e., combusting) a hydrocarbon fuel, often different than the alkane to be dehydrogenated, with oxygen in a furnace or other vessel, resulting in increased costs due to increased initial capital investment and ongoing fuel consumption.
European Patent Application Publication No. EP 1112241 (“EP '241”) describes a process designed to address this issue. Rather than burning a separate fuel to produce heat, the disclosed process involves combusting a portion of the alkane which is to be dehydrogenated, with oxygen, in the presence of a suitable combustion catalyst, to produce a heated stream containing the products of combustion (i.e., carbon oxides and water), unconsumed oxygen and unconsumed alkane. The heated stream is fed directly to an endothermic catalytic dehydrogenation reaction stage where the unreacted alkane is converted to the corresponding alkene in the presence of a suitable dehydrogenation catalyst.
More recently, International Patent Application Publication No. WO 2004/054945 (“WO '945”) provides an improvement to the aforesaid two-stage exothermic-endothermic process, which eliminates the need for the combustion catalyst by substituting an ignition source, such as a pilot flame or a spark ignition, and burning a portion of the alkane to produce a heated stream comprising unreacted alkane, and either products of combustion (i.e., carbon oxides, water and heat), or synthesis gas (i.e., carbon monoxide and hydrogen).
Thus, in the processes of both EP '241 and WO '945, the need to burn a separately provided hydrocarbon fuel to preheat the alkane feed is avoided. However, a portion of the alkane reactant is consumed, which leaves less available for conversion to the desired product in the dehydrogenation stage. Furthermore, products of combustion are incidentally formed, which increases the amount of unwanted by-products, without any contribution to the quantity of the desired alkene product. In fact, when a portion of the alkane reactant itself is burned, as taught by these sources, a diminished amount of alkane remains available for the dehydrogenation reaction and less of the desired alkene product is produced.
Accordingly, notwithstanding the work conducted to date in this field, industry continues to grapple with the aforesaid problems of increasing overall production of alkene (i.e., increasing alkene selectivity and yield), while minimizing the costs of dehydrogenation of lower alkanes to their corresponding alkenes. Development of an improved process and catalyst system for converting an alkane to its corresponding alkene, which provide improved selectivity and yield of the desired product alkene would be welcomed by industry. It is believed that the processes and catalysts of the present invention address these needs.